Conversion lenses for changing the focal length of a main lens system of a photographic camera are known in the prior art. Those designed for mounting in front of a main lens system are called front conversion lenses and those designed for mounting in back of a main lens system are called rear conversion lenses.
Examples of rear conversion lenses are described in Japanese Laid Open Patent Applications S63-200113 and S63-301916. The rear conversion lens described in Japanese Laid Open Patent Application S63-200113 extends the focal length of the main lens system by a factor of about 1.4 using a four lens group, five lens element construction. This conversion lens provides excellent optical performance for cameras that use 35 mm film. The rear conversion lens described in Japanese Laid Open Patent Application S63-301916 extends the focal length of a main lens system by a factor of 2.0 using a six lens element construction. This conversion lens also provides excellent optical performance for cameras that use 35 mm film.
However, although the rear conversion lenses as described above provide excellent optical performance using 35 mm film, there is a problem if such a lens is used with a camera that uses film of an even larger size, such as BROWNIE film size (6 cmxc3x974.5 cm). More particularly, such conversion lenses provide insufficient optical performance for cameras that employ BROWNIE film size, where the focal length must be extended by a factor of about 1.7.
The object of the present invention is to provide a rear conversion lens which enables the focal length of a main lens system to be extended by a factor of 1.5 to 2, while providing good optical performance for cameras that use a film size larger than 35 mm film, such as BROWNIE film size (6 cmxc3x974.5 cm) or larger.